1. Field of the Invention
The present invention relates to a detector which employs a ferro-magnetic thin film resistor element (hereinafter called "magneto-resistive element") using a magnetic anisotropic effect to detect the position of a linearly moving object or the rotational angle of a rotating object.
2. Description of the Prior Art
FIGS. 1(A) and 1(B) are a plan view and a front elevation showing an example of a magnetic-resistive element.
In FIGS. 1(A) and 1(B), the magneto-resistive element 2 and the terminals 3, 4 for external connections are formed on the surface of the substrate 1. The resistance of the magnetic-resistive element 2 changes according to the change of the intensity of the magnetic field in the direction of .+-.X shown in FIG. 1 whereas said resistance hardly changes in response to the change of the intensity of the magnetic field in other directions, and has what is called a magnetic anisotropic effect.
FIG. 2 shows a characteristic curve of said magneto-resistive element representing the change in the resistance of said magneto-resistive element with respect to the change of said intensity of the magnetic field in the direction of .+-.X.
In FIGS. 2, a, c, and e are the regions where the change of the resistance with respect to the intensity of the magnetic field is non-linear and b and d are the regions exhibiting a relatively good linear relationship in regard to the change of the resistance with respect to the intensity of the magnetic field.
The detector which utilizes the characteristic of the magneto-resistive element explained in FIG. 1 and FIG. 2 will hereinafter be described.
FIGS. 3(A) and 3(B) shows an example of a conventional detector which uses a magnetic-resistive element: FIG. 3(A) is a plan view, and (B) is a sectional view taken along line A--A of (A). In FIGS. 3(A) and 3(B), the same parts shown in FIG. 1 are represented by the same numbers.
Now, this conventional detector detects relative positions of both a movable element 8 and a stator 9 as the movable element 8 moves linearly along the stator 9, and the movable element 8 consists of a substrate which forms the magneto-resistive element 2 and terminals 3, 4 on the surface thereof and a permanent magnet 7 which functions to supply the bias magnetic field. On the other hand, the stator 9 includes on the plane opposite to the movable element 8 a large number of teeth like salients and slots of magnetic material (hereinafter called magnetic teeth). The opposing planes of the movable element 8 and the stator 9 are not illustrated, however, both planes are held by a travelling support system such as one using bearings so that a fixed clearance is maintained for maintaining the detector and the stator 9 parallel.
As shown in FIG. 3(A), it is generally known that when the magneto-resistive element 2 is connected through the terminals 3, 4 to an external constant-voltage regulated power supply 10, a fixed resistor 5, a ground 11, and an output terminal 6 and the movable element 8 moves along the stator 9 at a constant speed, an output like sinewave having half of the pitch of the pitch of the magnetic teeth of the stator 9 is obtained from the output terminal 6. This is because that the magnetic field generated by the permanent magnetic 7 is affected by the shapes of the magnetic teeth of the stator 9 to be bent cyclically thereby generating a cylindrical component in the magneto-sensitive direction (.+-.X direction) with respect to the magneto-resistive element 2 of the movable element 8. As explained in FIG. 2, the resistance of the magneto-resistive element 2 changes according to the affected intensity of the magnetic field and thus there is a cyclic output of resistance in the form of a sinewave generated at the output terminal 6.
This condition is further described in detail using FIG. 4, and the same parts shown in FIGS. 3(A) and 3(B) have the same numbers while the redundant descriptions are omitted.
In FIG. 4, 101, 102, 103, 104, and 105 represent typical lines of magnetic force generated by the permanent magnetic 7. In FIG. 4, the magneto-resistive element 2 is at a position through which the line of magnetic force 103 passes and since the line of magnetic force 103 passes through the magneto-resistive element 2 vertically there will be no component in the direction of .+-.Z, and said line of magnetic force 103 is at the zero point of the magnetic field shown in the characteristic curve of FIG. 2 indicating that the resistance of the magneto-resistive element 2 is at a maximum. The maximum resistance is also indicated when the magneto-resistive element 2 is at positions where the lines of magnetic force 101 and 105 pass in FIG. 5 as the movable element 8 is moved.
Now, if we consider that the movable element 8 has moved and the magneto-resistive element 2 comes to the position of the line of magnetic force 102, the line of magnetic force 102 will have passed through the magneto-resistive element 2 diagonally.
By dividing this line of magnetic force 102 into orthogonal components, a component which passes vertically through the magneto-resistive element 2 and a component in the -X direction are obtained and the resistance of the magneto-sensitive part varies with the intensity of the magnetic field component in the -X direction.
Likewise, a magnetic field in the +X direction is generated by the line of magnetic force 104. As shown in FIG. 2, since the characteristic of the magneto-resistive element 2 shows a symmetrical shape centered around the zero point of the magnetic field, when the movable element 8 moves at a constant speed and the magneto-resistive element 2 moves from the position of the line of magnetic force 101 to that of the line of magnetic force 105 (for one cycle of the magnetic teeth), the change of the resistance during that time will be symmetrical from 101 to 103 and from 103 to 105 with respect to the position of the line of magnetic force 103 as a center.
In other words, an output of half the pitch of the magnetic teeth can be obtained. However, in a detector having a construction as above, since the characteristic of the magneto-resistive element shown in FIG. 2 is used in both the plus and minus directions with respect to the zero point of the magnetic field as a center, such a detector has a defect in that the output signal is considerably distorted by the effect of the non-linear region c. Especially, when a highly accurate detector capable of fine resolution is necessary, a reduction in pitch of the magnetic teeth of the stator 9 can enhance this detector capability to a certain extent on one hand but signal detection becomes difficult on the other hand. Therefore, an alternative for obtaining fine resolution of signals is also achieved in many cases by generally employing a method such as electrical interpolation. In such a case, it is desirable to have the output wave-form similar to the sinewave from the standpoint of the signal processing but it is difficult to materialize fine resolution by a conventional detector because of poor linearity caused by said distortion of the output signal.
Since the defect of the conventional detector as described above is caused by the use of the non-linear region c of the characteristic of the magneto-resistive element shown in FIG. 2, it is apparent that the region b or d having a relatively good linear characteristic shown in FIG. 2 should be used in order to remove this defect.
To do so, it is necessary to always apply a constant DC magnetic field (the center of the region b or the center of the region d in FIG. 2) having a +X or -X direction to the magneto-resistive element 2 in FIG. 1 though it is difficult to supply such a DC magnetic field (hereinafter called as the offset magnetic field) stably.